Various types of food service systems and conductive heating rethermalization methods are known in the prior art. A most basic food service system is one in which food is served immediately, or only a short time period, after the food is cooked or prepared. Typically in better restaurants, food is served immediately after it is cooked or prepared. Such immediate service of the food generally results in the best quality of food. In other environments such as fast food restaurants and cafeterias, food is not served immediately after preparation, but rather kept warm and served in the warm state. In fast food restaurants, food such as hamburgers is kept hot in disposable plastic containers. In cafeterias, food can be kept warm in the bulk state and served to individuals on a request basis, or food can be prepared in bulk and, thereafter, divided into individual portions and kept warm in a pellet system. A pellet system can also be used in non-cafeteria environments, such as hospitals wherein the food is placed on serving dishes and covered with insulated domes to keep the food warm until service.
A significant drawback of systems which keep food warm for a short period of rime before service is that the rime period during which the food can be kept safe and warm is relatively limited, and the quality of the food deteriorates over rime.
A rethermalization system is another prior art type of food service system. In a rethermalization system, food is cooked or prepared and then chilled in bulk to 40.degree. for less. Eventually the food is divided into individual portions and stored in a chilled state until it is reheated. The United States Food and Drug Administration (FDA) guidelines specify that the chilled and stored temperature must be 40.degree. F. or less. A short time before the food is to be served, the food which is to be served hot is brought to a safe serving temperature, i.e. it is rethermalized. FDA guidelines specify that a safe serving temperature is 165.degree. F. or greater. The present invention is directed to certain improvements in structural and functional aspects of a rethermalization system which uses conductive heating.
One manner of classifying prior art conductive heating rethermalization systems is by the location of the heating elements within the system. That is, prior art rethermalization systems have located the heating element in either a food serving dish, a dish supporting tray, or a shelf attached within a service cart.
U.S. Pat. No. 3,908,749 to Williams discloses a food service system wherein precooked foods are held on trays within a chilled environment cart. Food to be rethermalized is held within a dish which has an electrical resistance heating element embedded in its base. Contact buttons to connect the heating element to an electrical power source extend from the bottom of the base of the dish. The dish extends through a hole in a food service tray, and when the tray is supported in the service cart, the contact buttons rest on exposed leaf contacts which extend from a rear wall of the service cart.
Embedding the electrical heating element within the dish significantly increases the cost and complexity of the dish. A dish within a food service system undergoes severe handling since it must carry food, is subjected to heat for rethermalizing the food, and thereafter is subjected to heat, chemicals and handling impact in washing and drying processes. Thus, the dish is the component of this system which must be replaced most frequently. A system which incorporates the heating element into the dish thus has a relatively high long-term operating expense. Another disadvantage of this type of food service system is that relatively large electrical contacts must remain exposed in the cart to provide the electrical connection to the contacts of the dish. Such exposed contacts are thus subject to corrosion and water damage during operation and cleaning.
Food service rethermalization systems which incorporate heating elements into the service tray have disadvantages similar to dish heating element systems. That is, the cost of the trays is relatively high and the trays are subject to breakage because of frequent handling during food service and cleaning. Exposed contacts are also required to provide electrical connection to the heating elements embedded in the trays. Another disadvantage of tray heating element systems is that the trays become warped after a period of time due to the frequent heating and cooling cycles to which the trays are subjected and their relatively large planar configuration. Once the trays become warped, good surface contact between the heating element portion of the tray and the dishes to be heated is lost. As a consequence, accurate, and possibly adequate, heat is not applied to the food during rethermalization. Examples of food service rethermalization systems which utilize trays incorporating heating elements are found in U.S. Pat. No. 4,068,115 to Mack et al.; U.S. Pat. No. 4,167,983 to Seider et al. and U.S. Pat. No. 4,235,282 to deFilippis et al.
Food service rethermalization systems which incorporate the heating elements into shelves supported in the service cart alleviate the problem of the high cost of the more replaceable portions of the system, i.e. expensive dishes and trays. Furthermore, since the heating elements are formed as a portion of the cart, i.e. a shelf within the cart, the connection of the electrical heating elements to a power source is incorporated within enclosed portions of the shelf and cart, alleviating the problem of exposed contacts. However, rethermalization carts with shelf located heating elements do have certain limitations or disadvantages. Such systems generally incorporate a plurality of heating elements in a fixed manner into a single shelf. Thus, when one of the heating elements on a shelf becomes inoperative, the entire shelf must be replaced, rather than replacing the single inoperative heating element. Examples of food service rethermalization systems wherein the heating elements are incorporated into shelves within the service cart are found in U.S. Pat. No. 4,346,756 to Doddet al.; U.S. Pat. No. 4,323,110 to Rubbright et al. and U.S. Pat. No. 4,285,391 to Bourner.
Prior art rethermalization food service carts have one or more columns of vertically spaced storage shelves on which trays carrying dishes of food are held. Dependent upon whether the tray at a given shelf location is carrying food to be heated, the heating elements at the respective shelf locations have to be activated at an appropriate time. Power to all of the heating elements is generally turned on manually or via an automatic timer program. However, separate activation of the individual heating elements at the shelf locations is dependent on whether or not food to be heated is present at the shelf location. One technique is to have the food service personnel activate a manual switch as the food tray is loaded on the shelf, if the tray contains food to be heated. However, such a technique is labor intensive and requires that the food service operator inspect the food on the tray or otherwise have an indication of whether food to be heated is present on a tray being placed onto the shelf.
Rubbright et al. '110 discloses a programmable system for activating heating elements at various shelf locations. In this system, a transport modular pack is programmed to have each heater element follow a particular and independent time/temperature curve dependent upon the food to be heated at the particular shelf location. This technique is very complicated and labor intensive, and requires special care and attentiveness by a skilled operator, since a specific time/temperature curve must be programmed by the operator into the modular pack for each individual meal.
Another prior art technique for activating the individual heating elements at the shelf locations relies upon tray positioning. In Seider et al. '983, electrical heater contacts on the dishes mate with projecting electrical power contacts at the shelf locations when the tray is positioned in one direction, but do not make electrical contact when the tray is positioned in the 180.degree. opposite direction. In Bourner '391, a mechanical switch is placed at each shelf location and is activated by a tray cover when the tray is positioned in one direction, but is not activated when the tray is positioned in the 180.degree. opposite direction, because of a cut out formed in the cover. Again, care must be taken by the food service operator in positioning the trays within the shelves. Frequently the loading of the shelves with the trays occurs at a separate location, or is attended to by a different person, from the loading of the food onto the tray. Thus, miscommunication or misunderstanding as to the food located on the tray can result in incorrect positioning of the tray.
Electrical conductive heaters in prior art rethermalization systems and rethermalization methods using such heaters have exhibited certain limitations or drawback in the manner in which the food is rethermalized. Most typically, a rethermalization system utilizes a resistance heater, which operates intermittently at a single power level during rethermalization, i.e. during the time when the food is initially brought from the chilled state to a serving temperature. The resistance heater is controlled by an on-off thermostat which operates between upper and lower temperature of limits. Typically, the resistance heater applies heat over a predetermined time period. While such a heating method has worked satisfactory, it is subject to certain limitations. For example, the system must be designed to work within a practical time period. While it would be desirable from a labor/cost standpoint to rethermalize chilled food as quickly as possible, it is difficult to achieve high quality food when rethermalization occurs too quickly. For example, certain portions of food may become scorched while others remain cold, or the food may become dehydrated or discolored when rethermalization occurs too rapidly.
One commercial rethermalization system accomplishes rethermalization of food in approximately one quarter of an hour. However, the range or variety of food which can be rethermalized is limited and special plating techniques are required for many of the lighter or more fragile foods. The: term "plating techniques" refers to the special way or manner in which certain foods, which are to be rethermalized, are placed on dishware to assure that the foods retain their quality after rethermalization. Plating techniques are most frequently used with light and fragile foods. Examples of plating techniques include supporting food on a dish within a dish or on toast, adding gravy or water to the food, or spraying the dishes with a nonstick coating. Thus, while the time during which rethermalization takes place is less, any operating expense savings is very likely lost by the added expense is required in the special plating techniques.
Conversely, when the time for rethermalization is significantly increased, for example, to one hour as suggested in Williams '749 for an entree of a meat and one or two vegetables, the need for special plating techniques is reduced and a wider variety of food can be rethermalized. However, a one hour rethermalization time period can be too long when three meals per day must be prepared in typical institutional environments such as hospitals, prisons or nursing homes since scheduling options for handling and recyling the carts for the next meal are limited.
The system disclosed in Bourner '391 rethermalizes chilled food in approximately a one-half hour time period. Such a rethermalization time period has proven to be a satisfactory compromise. That is, a relatively wide variety of food can be rethermalized without the requirement of using special plating techniques. Furthermore, the one-hag hour time period has not proven to be too restrictive on service personnel, allowing sufficient time for preparation and service of three meals per day in an institutional environment.
Nevertheless, even the system disclosed in Bourner '391 has certain limitations. For example, an unrestricted range of foods cannot be rethermalized without special plating or quality degradation. It is difficult to rethermalize small portions of low density, fragile foods, while retaining high quality of the foods and at the same time rethermalize large portions of high density, difficult to heat foods within the same rime period. It is believed that one of the causes of this limitation is that the thermostat which controls the heater must operate at a relatively high temperature range in order to sufficiently heat the more difficult to rethermalize food, and that at such higher temperature ranges, the easier to heat foods become overcooked.
A system which utilizes a resistance heater at a single power rating also has proven unforgiving when certain set parameters are varied. For example, if portion sizes are varied too much from specified portions, quality of the reheated food deteriorates. This is particularly true if too much of a dense food is placed in combination with too little of a fragile, low density food. Such a system is also very voltage dependent. Thus, if the voltage supplied to the heating elements drops significantly below the norm, for example more than 5%, insufficient power may be supplied by the heating elements to heat the higher density, more difficult to heat foods. Similarly, if the voltage supplied to the heating elements increases excessively, for example 10%, excessive heat may be supplied to the more fragile foods causing deterioration in the quality of these foods.
The system in Doddet al. '756 uses a separate low power holding circuit for keeping food warm after it has been rethermalized. A higher power primary heating circuit is thermostatically controlled and used to rethermalize the food over a predetermined time period. Thus, while this system uses separate heaters having different power ratings, only the higher power heater is used for rethermalization, while the lower power heater is used primarily to keep the already rethermalized food warm. A switch is used to select between the two heating options.
The system in Mack et al. '115 uses PTC power heaters as the preferred type of heater embedded in the dish. The use of a resistance heater controlled by a thermostat is mentioned as an alternative to the PTC heater. The PTC heaters operate basically as multimodal resistance heaters so that below their critical temperature they have a low electrical resistance, while above it the resistance is very high. Near the critical temperature, the resistance varies between the extremes. Therefore, depending upon temperature a PTC heater will deliver different wattages. During the development of the present system, it was found that PTC heaters had both structural and functional disadvantages. The heaters initially draw very high current, requiring more expensive high power capability electrical service. Also, if the initial input temperature of the food varied from preset standards, the heaters would not adequately heat the food to proper serving temperature.
The Rubbright et al. '110 system programs an individual time/temperature curve for each heater element. A predetermined time period is not used for all of the types of foods to be rethermalized. Rather, a particular time/temperature curve is used to control heater temperature and time independently for each heating element depending upon the type of food being rethermalized. A certain number of programs are available for use and are coordinated with various types or combinations of foods. The food service operator thus must coordinate the particular food or combination of foods with the appropriate program. The food service handling process is thus complicated, requiring extra care and attention during meal preparation and special training for the operators of the system.
The food service system and method of rethermalization of the present invention was developed to overcome the structural and functional limitations of the prior art systems and methods discussed above.